1. Field of the Invention
The present invention relates to a radio communication system, such as, a portable radio telephone system or a land mobile radio telephone system. More particularly, the present invention relates; to a current control circuit applied to each of a high-frequency amplifier circuit and a frequency converter circuit in a radio communication system for operating with low current consumption.
2. Description of the Related Art
FIG. 4 shows the basic structure of one form of a transistor amplifier circuit used in a prior art radio communication system. Referring to FIG. 4, the transistor amplifier circuit includes a transistor 2, bias setting resistors R4, R6, R8 and an emitter resistor R10.
In the transistor amplifier circuit having the structure shown in FIG. 4, the level of its power supply voltage Vcc is changed to control the collector current Ic of the transistor 2. Also, the resistance value of the emitter resistor R10 is changed to control the emitter current Ie of the transistor 2. By suitably changing the level of the power supply voltage Vcc and the resistance value of the emitter resistor R10, the current consumption of the transistor amplifier circuit can be determined.
However, the transistor amplifier circuit used in the prior art radio communication system is defective in that, when the collector current Ic or the emitter current Ie of the transistor 2 is directly controlled, the amplifier operating point tends to fluctuate, resulting in impedance mismatching.
Thus, in order to avoid the undesirable fluctuation of the operating point of the transistor amplifier circuit in the prior art radio communication system, a current control circuit having a complex structure and consuming a relatively large current is inevitably required.